The interference effect produced by two different interference methods
Electromagnetic interference (EMI) includes conduction interference and radiation interference. Conducted interference refers to the coupling (interference) of signals on one electrical network to another electrical network through a conductive medium. Radiation interference refers to the interference source coupling (interference) its signal to another electrical network through space. In high-speed PCB and system design, high-frequency signal lines, integrated circuit pins, various connectors, etc. may become radiation interference sources with antenna characteristics, which can emit electromagnetic waves and affect other systems or other subsystems in the system. normal work.
"Common mode" jammer refers to the interference between lines (including power lines and signal lines) to the earth. For power lines, it refers to the interference between the live wire and the earth or the neutral wire and the earth. For three-phase circuits, common mode interference exists between any phase and the earth. Common mode interference is sometimes called longitudinal mode interference, non-pair interference, and ground interference. This is the interference between the current-carrying conductor and the earth.
"Differential mode" portable jammer is the interference between lines (including between power lines, signal lines and its ground return line). For power lines, differential mode interference refers specifically to the interference between the phase line and the neutral line; for three-phase circuits, differential mode interference also refers to the interference that exists between the phase line and the phase line. Differential mode interference is sometimes called normal mode interference, transverse mode interference or symmetrical interference. This is the potential difference between current-carrying conductors.
The existence pattern of the jammer indicates the relationship between the interference source and the coupling path. For example, common mode interference indicates that the interference is coupled into the circuit by radiation or crosstalk. Such as lightning, electric arcs near the equipment, nearby radio stations, and other high-power radiating devices on the power line interference, as well as the interference of the internal wiring of the chassis or other cables to the power line. Since it is the induction from space (electromagnetic radiation, inductive coupling and capacitive coupling), the effect on each line is the same. The differential mode interference indicates that the interference originated in the same power line (direct injection). For example, motors, switching power supplies, thyristors, etc. working in the same line, the interference they produce on the power line is differential mode interference.
Usually, these two components of the interference voltage on the line exist at the same time, and due to the imbalance of the line impedance, the two components will change to each other during transmission. After the interference is transmitted over a long distance on the line, the attenuation of the differential mode component is greater than that of the common mode component. This is because the impedance between the lines and the line-to-ground impedance are different. On the other hand, the frequency of common mode interference is generally above 1~2MHz. Therefore, when common mode interference is transmitted on the line, it will also radiate to the surrounding adjacent space (this is because the line-to-ground impedance is large, plus common mode interference). The frequency is relatively high, so it is easy to escape the transmission line and form space induction). The radiation of the power cord, especially the radiation of the power cord after entering the device, can be further coupled to the signal circuit to cause interference, so it is difficult to prevent it. The frequency of differential mode interference is relatively low, and it is not easy to form radiation. In addition, in general circuits, many measures have been taken to deal with differential mode interference (for example, a large capacitor has been used in the voltage regulator circuit; on the printed circuit board, the power line and the ground line Decoupling capacitors are also commonly used), so the chance of equipment malfunction caused by differential mode interference is relatively less. Therefore, the sensitivity of the equipment is mostly caused by common mode interference.
In the current common mode jammer is the focus of our consideration, which can be confirmed from the commonly used immunity test content. Among them, electrostatic test, high-frequency radiation electromagnetic field test, electrical fast transient pulse group test, line-to-ground lightning surge test and conduction test caused by radio frequency field induction, etc., for the tested equipment and circuit, What they feel is common mode interference.